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Mars' great canyon complex, Valles Marineris, dwarfs the size and splendor of Earth's own Grand Canyon. But while geologists have a formed a fairly complete picture of how the Grand Canyon formed, the mechanisms that carved out Valles Marineris and its component canyons have been a longstanding mystery, with explanations ranging from massive floods to tectonic processes like those that cause earthquakes and build mountains on Earth.

"How did these gigantic canyons really form? Were they all formed by floods, or were other things going on?" asks John Adams of the University of Washington in Seattle and lead author of a new study that seeks to answer the questions. "These have been controversial questions going back to the very first Mariner pictures of Mars. And they're still controversial questions, which means we don't really fully understand what's going on yet."

The answer for how at least parts of the canyon complex formed may lie in Hebes Chasma, a 190-mile-long (310-kilometer) scar cut into the Martian surface and connected to the main body of Valles Marineris.

Adams and his colleagues, Alan Gillespie and David Montgomery, think that Hebes, and other chasms, might have formed after salts in the surface layers were heated up, causing water to melt out below the surface. This water then rushed out through underground plumbing, causing the layers of dirt and rock above to collapse, creating the canyon scar in the surface.

Heating salts
Montgomery and Gillespie began with the observation of sulfate salts in the area of the canyons. (These compounds have been found in numerous spots around the Martian surface by orbiting spacecraft and robotic rovers.)

These salts are hydrated, which means that molecules of water are attached to the sulfate molecules. Montgomery and Gillespie thought about what would happen if those hydrated salts were heated up. The physics and chemistry involved in the process indicated that the water would separate out from the salts, taking with it some dissolved materials. This muddy, briny water would pool up underground, causing an increase in volume under the surface.

The heating of these salts on Mars isn't a stretch, "because it gets hotter as you go down with depth of course," Adams explained. "But more importantly, the huge canyon complex of Mars is right next to the huge volcanic complex, the Tharsis plateau." The heat involved with volcanic processes could have warmed up the salts in the Martian regolith.

Once the muddy water was separated, it would have to escape somehow from its subterranean well because there wasn't enough room under the dirt and rocks to hold all that newly formed water. The two possibilities for the water's escape were: bursting out over the surface; or funneling out below it, like a plug pulled from a bathtub drain.

The scientists needed a place to test out the drain part of the theory, which is where Hebes Chasma came in.

No way in or out
Hebes Chasma was of interest because it is "the very best example of a fairly large canyon in the Valles Marineris complex that has absolutely no inlet or outlet on the surface," Adams told SPACE.com. "There's no way that water could have gone out and over the edge there." So Hebes Chasma could not have formed by erosion from giant floods, like the carving of the Grand Canyon but on a larger scale.

Montgomery and Gillespie and other researchers looked to see if the landforms found in Hebes Chasma were what would be expected of a situation in which the muddy brine drained out of it underground pool through a series of subsurface channels, like pipes. Adams and his colleagues made detailed geologic maps of the region to see if the features were consistent.

They also enlisted the help of geologists at the University of Texas at Austin, who built a desktop-sized scale model of Hebes Chasma and filled it with silicon gel and sand mixtures, then drained it to see what would happen.

"The results of that were absolutely astounding to us," Adams said — they matched the formations found in Hebes Chasma to a T.

The match of the two approaches led Adams and his colleagues to conclude that Montgomery and Gillespie's process likely explained the formation of Hebes Chasma.

There are, of course, still plenty of unanswered questions.

More drained chasms?
For one thing, the scientists don't know exactly when Hebes formed or how long the process took.

Adams said the chasm likely formed early in Mars' history, with the process stopping several billion years ago.

"It may have taken a few million years, or a hundred million years, we just don't know the answer to that," Adams said.

It's possible that the briny mixture traveled underground and came up and out elsewhere — a possible source of the outflow floods thought to have formed other parts of the canyon.

Adams and his colleagues plan to look closely at more chasms that also show evidence of material sliding down into depressions in the surface to see if they can also be explained by this mechanism.

How much of Valles Marineris might have formed from the drainage of muddy brines isn't known, and it's likely that the whole system formed from a "mixed bag" of mechanisms, including floods, drainage and tectonic forces, Adams said.

The new study was detailed in the August issue of the journal Geology.